1. Field of the Invention
The present invention relates to a motor-driven, magnetically coupled pump which is used as an automotive water pump or the like.
2. Description of the Prior Art
A known magnetically coupled pump of this kind is described in Japanese Patent Laid-Open No. 591/1984 and comprises a pump housing whose interior is partitioned into a pump chamber and a motor chamber by a partition wall. The stator of an axial air-gap motor is mounted in the motor chamber. A rotating shaft is pivoted to the center of the stator and protrudes into the pump chamber. An impeller is mounted to the front end of this protruding shaft. An annular permanent magnet alternately magnetized south and north circumferentially is mounted on the stator side of the impeller which faces the partition wall.
In this magnetically coupled pump, exciting currents are supplied to a plurality of stator coils mounted on the outer surface of the stator to produce rotating magnetic fields at the end surfaces of the stator cores. Thus, a magnetic attraction acts between the stator and the annular permanent magnet on the impeller. Consequently, the impeller is rotated. However, the impeller equipped with the permanent magnet and acting as a rotor has the cantilevered rotating shaft. A slight clearance exists between the rotating shaft and its bearing. Also, the impeller is not locked against radial swinging movement. Therefore, if the impeller rotates with poor uniformity, the impeller tends to be vibrated. Furthermore, if the rotating shaft swings, noise is induced. This also causes rapid wear of the bearing.
In this kind of magnetically coupled pump, the magnetic attraction acting on the impeller contains a large thrust-direction attracting force component directed from the permanent magnet of the impeller toward the stator cores. Therefore, during rotation, the impeller portion at the top of the rotating shaft is pressed against the bearing of the rotating shaft with a large force. Consequently, a flat washer or the like is mounted between the impeller-holding portion of the rotating shaft and the end surface of the bearing to reduce the friction between them.
However, if fine and hard foreign matter intrudes into the pump chamber and enters the space among the end surface of the bearing, the flat washer, and the impeller-holding portion, then these components are quickly worn out. Finally, the impeller comes into contact with the partition wall inside the pump chamber. This lowers the rotating speed of the impeller. In consequence, the flow rate of the pump decreases. As a result, the life of the pump is shortened.